Fibrosis of the iliopsoas muscle can result in pelvic limb lameness in dogs. In this case report we describe fibrosis after an initial injury of the iliopsoas muscle in a dog. A seven-year-old hunting dog developed an acute onset of lameness and pain of the left pelvic limb after an intense period of exercise. Two months later, the dog was referred for evaluation of a non-weight-bearing left pelvic limb lameness. Orthopaedic examination revealed pain on hyperextension and internal rotation of the left coxofemoral joint. Neurological examination revealed a decreased patellar reflex in the left pelvic limb. Computed tomographic images showed non-uniform, contrast enhancement of the left iliopsoas muscle. The animal was treated with an iliopsoas tenomyectomy. Histopathological examination of the affected iliopsoas muscle revealed endomysial and perimysial mature fibrous replacement tissue. Sixteen weeks after surgery, the dog had returned to pre-injury levels of exercise.